Caption: Spirit level. Silhouette of a spirit level used to measure how horizontal a surface is. It consists of a tube filled with fluid except for a single bubble. When the spirit level is placed on a horizontal surface, the bubble remains between the two vertical lines in the clear part of the tube (as here). However, the bubble will move along the tube and beyond the markings if it is not placed horizontally. Spirit levels are widely used in the construction industry to ensure that structures have been correctly aligned.